Inkjet printers are now very common and affordable and allow one to obtain decent print quality. They are used in home printing, office printing and commercial printing. In inkjet printers, print heads are used to eject ink droplets very accurately to place them on a desired location on a medium. The print head normally comprises a large number of nozzles, often, more than 400 nozzles. As a general rule, the larger the number of nozzles, the greater is the improvement of the print quality and speed. Frequently, the nozzles become blocked because of the usage of pigmented inks or inks containing particles. Sometimes, one or more nozzle orifices is may contain dried ink and fresh ink cannot be ejected. One result of this condition is the formation of streaks, which lead to poor print quality.
Near-infrared (near-IR) absorbing dyes may be added to the ink to monitor the condition of the nozzles. Other applications for such dyes exhibiting absorption in the near-IR include security printing, counterfeit assessment, RFID tags, etc. Near-IR absorbing dyes may be employed by extending the conjugation so that the absorption can be shifted to the range of 700 to 1000 nm range.
Many of the technologies utilizing near-IR materials require that these materials be dissolved in organic solvents, water and aqueous or organic solvent blends. Some applications, such as thermal inkjet printing require that the near-IR absorbing material be kept in aqueous solution for long periods of time. Near-IR dyes have to be stable in aqueous solvent blends for a long time without undergoing any kind of chemical change. Any degradation or changes to its physical/chemical nature can destroy the conjugation and thus lose the near-IR absorption. Such changes could adversely affect the desired property (absorption in the near-IR wavelength range) and cannot be used in such applications. Near-IR dyes of the Cyanine dye class are soluble in water but are chemically unstable in aqueous solutions over long time at wide pH range and are therefore not suitable for inkjet applications requiring aqueous solution stability for long time.
Phthalocyanine (PC) and naphthalocyanine (NPC) (metal free or metal complex) are chemically stable but are difficult to solubilize in water. Highly ionic groups or water-soluble ethylene oxide groups have to be attached for dissolving these compounds in water. These compounds are solubilized by attaching water-soluble groups on the benzene rings. But introduction of such groups changes the peak absorption dramatically up to 60 nm.